The present invention relates to a color picture display apparatus employing an array of light-emitting elements such as light-emitting diodes (LEDs).
LED panels are used as components of, for example, large outdoor screens on which color television pictures and other color pictures are displayed. A single picture element or pixel may consist of a single red, green, or blue LED, or of a group of three or more LEDs of these primary colors. The LEDs of different colors are arranged in a regular pattern, but are not necessarily present in equal numbers. One known type of display apparatus, for example, achieves enhanced resolution by employing a one-LED-per-pixel array with extra green pixels, taking advantage of the fact that the human visual system is most sensitive to green light.
A problem encountered in LED-panel display apparatus is that the colors emitted by red, green, and blue LEDs differ from the colors emitted by the red, green, and blue phosphors employed in a conventional television picture tube or cathode-ray tube (CRT). An LED panel can display a wider range of colors than can a CRT. Consequently, when a standard television picture is displayed by apparatus employing LED panels, the picture has an unnaturally gaudy appearance.
Another problem is that individual LEDs, particularly green LEDs, vary in their spectral emission characteristics. These variations lead to erratic color rendition, which can degrade picture quality fairly seriously. The variations can be reduced by screening the LEDs and selecting LEDs with uniform characteristics for each primary color, but the screening process increases the cost of the apparatus. A conceivable alternative is to compensate for the individual LED differences, but while it is fairly easy to compensate for luminance variability, but it is generally difficult to compensate for spectral variability, i.e., color variability.
In a one-LED-per-pixel display, a further problem occurs due to the physical separation between pixels of the same primary color. If a nearly monochromatic red or blue picture is displayed, for example, the picture has a grainy appearance because of the mutual separation of the red or blue pixels. This is especially true in the above-mentioned apparatus with extra green pixels, because the red and blue pixels have to be widely separated to accommodate the increased number of green pixels.
LEDs are not the only light-emitting elements used in large-screen display apparatus. A large screen can be constructed from an array of CRTs, for example, in which case the light-emitting elements are phosphor dots. It would be desirable to solve the problems described above in a way that would be applicable to screens with any type of light-emitting elements.
An object of the present invention is to provide a color picture display apparatus having signal-processing circuitry capable of generating a natural picture appearance with any type of light-emitting elements.
Another object of the invention is to reduce the degree of picture degradation caused by variations in the spectral emission characteristics of the light-emitting elements.
A further object is to avoid a grainy picture appearance.
The invented color picture display apparatus comprises means for converting a color television signal to a plurality of monochromatic digital signals, means for modifying the monochromatic digital signals so as to obtain desired hues, and means for displaying a color picture according to the modified monochromatic digital signals by use of a regular array of light-emitting elements, each emitting light of a color corresponding to one of the monochromatic signals. The emitted colors are, for example, red, green, and blue.
The monochromatic signals are modified by, for example, a matrix multiplication operation using externally programmable coefficients.
If the light-emitting elements are LEDs, and if the monochromatic signals are sampled at different timings, corresponding to the positions of the LEDs in the array, so that each LED constitutes a separate picture element in the picture, then the array preferably has a comparatively large number of long-wavelength LEDs, or red LEDs.